Ultrasonic intrusion detection system signal processing circuit

ABSTRACT

An electronic device for processing the amplitude modulations of a received ultrasonic carrier wave to produce an alarm signal when the pattern of the amplitude modulations is characteristic of an alarm condition. The device produces an electrical signal which is representative of the amplitude modulations of a received wave. When the electrical signal has a frequency within a predetermined pass-band, is of at least a predetermined amplitude, and persists for a minimum time interval, an alarm signal is produced.

United States Patent Lee et al. May 9, 1972 [54] ULTRASONIC INTRUSIONDETECTION 2,782,405 2 1957 Weisz et al ..340/258 SYSTEM SIGNALPROCESSING 3,258,762 6/1966 Donner ..340/258 X CIRCUIT PrimaryExaminer-John W. Caldwell [72] inventors: John H. Lee, Township ofWoodbury, A i-Ymm E.\'aminerMiChael Slobasky Washington County; ThomasE. Collins, Attorney-Kinney, Alexander, Sell, Steldt & Delahunt EastOakdale, both of Minn. [73] Assignee: Minnesota Mining and Manufacturing[57] ABSTRACT p y, l, Minn. An electronic device for processing theamplitude modulations of a received ultrasonic carrier wave to producean alarm [22] Flled' 1969 signal when the pattern of the amplitudemodulations is PP 73 characteristic of an alarm condition. The deviceproduces an electrical signal which is representative of the amplitude[52] U S cl 340/258 R 340/258 A 340/261 modulations ofa received wave.When the electrical signal has [51] G68) 13/00 a frequency within apredetermined pass-band, is of at least a [58] mi, 258A 261pmmmmmmmmmmmmm mm mmmmfmm mimmmm mm terval, an alarm signal is produced.

[56] Rem-anus Cited 8 Claims, 3 Drawing Figures UNITED STATES PATENTS2,767,393 10/1956 Bagno ..340/258 PATENTEDMAY 9 I972 SHEEI 2 [IF 2 5E5 1s Mm 1M5 ,wA

ULTRASONIC INTRUSION DETECTION SYSTEM SIGNAL PROCESSING CIRCUITBACKGROUND OF INVENTION Systems for detecting conditions correspondingto a fire or an intruder within a protected space by sensing for changesin ultrasonic acoustic waves radiated into the space are well known inthe art. In such systems, ultrasonic acoustic energy of a fixedfrequency (the carrier frequency) is radiated into the space to beprotected. An acoustic energy receiver is stationed in the space. Whenthere is no movement within the space, the radiated acoustic energyexists as a standing wave pattern. Disturbances within the space, suchas those caused by an intruder, a fire, or environmental noise such asthe ringing of a telephone and air turbulences caused by exhaust fans,vehicular trafiic passing near the protected space or even vibratingwindow blinds or water pipes will disturb the standing wave pattern tocause a variation in the energy sensed by the receiver.

Many analyses have been made to determine the characteristic variationsof the different types of disturbances. A long sought after objective ofthese analyses has been to unambiguously distinguish between variationscharacteristic of a fire or an intruder and variations characteristic ofenvironmental noise.

Many such analyses, and systems predicated upon the conclusions of theseanalyses, considered the frequency composition of the received wave tobe the criteria for unambiguously identifying different types ofdisturbances. Such a prior art system shall hereafter be referred to asa frequency demodulated" system, i.e., one in which an electrical signalcorresponding to a received acoustic wave is frequency demodulated.

Such frequency demodulated systems are disclosed in US. Pat. Nos.2,655,645; 2,794,974; and 3,1 11,657. An objective of each of the lattertwo of these patents is to solve the problem of false alarms. A premiseof both patents is that the amplitudes of the different frequencycomponents of valid alarm signals (caused by a fire or an intruder) areessentially the same whereas the corresponding amplitudes of turbulencesignals vary inversely with frequency. These patents thus each includecircuits for separating a received signal into a low frequency componentand a high frequency component together with additional circuitry forcomparing the amplitudes of the two signals. A circuit is included forattenuating the low frequency component so that the comparison producesa difference signal only for valid alarm signals. The two components ofa turbulence signal, after attenuation of the low frequency component,are approximately equal and thus do not produce a difference signal whencompared. U.S. Pat. No. 3,11 1,657 also includes additional circuitryfor preventing false alarms when the composition of a turbulence signaldoes not simultaneously include both a high and a low frequencycomponent as is sometimes the case. This additional circuitry comprisesintegrators for time-averaging the high and low frequency components andwill prevent a turbulence signal having only a high or a low frequencycomponent from producing a false alarm for those cases when thetimeaverage of the high and low frequency components over a preselectedperiod is approximately equal.

We have discovered that it is not necessary to frequency demodulate thereceived signal. We thus are able to eliminate all of the circuitry forseparating a received signal into two components and for comparing andotherwise processing these components. Instead, we merely amplitudedemodulate received signals. We have found that the frequency, duration,and amplitude of the amplitude demodulation signal of a received wave isa reliable criteria for distinguishing between environmental noise andvalid alarm condition disturbances. Specifically, we have found thatsignals of relatively low frequency correspond to turbulencedisturbances whereas a higher frequency signal is characteristic of avalid alarm disturbance, i.e., both fire and intrusion typedisturbances. We

have also found that environmental noises other than turbulence, e.g.,the ringing of a telephone, are characterized by amplitude modulationsof still higher frequencies.

In a preferred embodiment, the ultrasonic motion detector device of thepresent invention comprises a circuit for producing an electrical signalcorresponding to amplitude modulations of a received ultrasonic wave.The received waves coma prise reflections of a wave both radiated into aspace to be protected and modulated according to disturbances within thearea. The electrical signal is applied to another circuit which producesan alternating signal. The alternating signal is produced to have afirst value (i.e. the difference between the high and low amplitudes ofa signal excursion) when the absolute value of the electrical signalamplitude is greater than a predetermined reference amplitude and tohave another value less than the first value when the absolute value ofthe electri' cal signal amplitude is less than the predeterminedreference amplitude. The alternating signal is applied to a furthercircuit which passes the alternating signal as an output signal when thefrequency of the alternating signal is within a predetermined pass-band.Another circuit receives the output signal and produces an alarm signalwhen the output signal corresponds to a train of alternating signalexcursions of the first value occurring within a predetermined orminimum time. By blocking passage of alternating signals of low and highfrequency and by requiring that an alternating signal which is passedpersist for a minimum time, the signal processing circuit of the presentinvention avoids false alarms while reliably indicating valid alarmconditions.

In another embodiment, an additional circuit is provided for effectivelyvarying the predetermined reference amplitude in direct proportion tochanges in the time average of the output signal amplitude to compensatefor changes in the ambient level of environmental noise signals.

Specific embodiments of the invention chosen for purposes ofillustration and description are shown in the accompanying drawingswherein:

FIG. 1 is a block diagram of an ultrasonic intrusion detector systememploying the processing circuit of the present invention;

FIG. 2 is a schematic circuit diagram of a preferred embodiment of thesignal processing circuit of the present invention;

FIG. 3 is a schematic circuit diagram of a circuit for use with theprocessing circuit of FIG. 2 to compensate for variations in the ambientenvironmental noise signal level.

With reference to the block diagram of FIG. 1 there is shown atransmitter 10 which radiates ultrasonic waves 12 at an essentiallyconstant amplitude and frequencyta carrier" frequency) into a space tobe protected. The transmitter 10 is not considered a part of the presentinvention. Reflected waves 14 impinge upon and are demodulated by areceiver l6. The output of the receiver is an electrical signal which isa representation of the amplitude modulations of the received wave. Anillustrative representation of the electrical signal produced byreceiver 16 is illustrated as waveform 18. The output of receiver 16 isshown to be applied to a saturation amplifier 20 which converts theelectrical signal 18 to an alternating signal, shown as waveform 22.Waveforms l8 and 22 have the same time base. Thus it is apparent thatthe saturation amplifier 20 provides a signal the excursions of whichhave a first amplitude whenever the electrical signal of waveform 18exceeds a predetermined reference amplitude. Otherwise, for amplitudesof waveform 18 less than a predetermined amplitude, the signal excursion(i.e., the peakto-peak amplitude) is less than the first amplitudeexcursion. In the waveform l8, zero reference is indicated by solid line24 and the predetermined amplitude is represented by the equidistantdisplacements of dashed lines 23 and 25. In waveform 22, the firstamplitude excursions, correspondingto waveform 18 signals greater thanthe predetermined amplitude limits of dashed lines 23 and 25, are shownto occur between reference lines 27 and 29. The alternating signal ofsaturation amplifier. 20 is applied to an active filter 26. Activefilter 26 passes as an output signal, shown as waveform 28, thosealternating signals having a periodicity within a predeterminedpass-band. The lower limit of the pass-band rejects turbulence typesignals; the upper limit rejects environmental noise signals like thosefrom a ringing telephone. The output signal 28 from active filter 26 isapplied to an alarm and indicator circuit 30 which produces an alarmsignal in response to an output signal corresponding to a train of firstamplitude alternating signal excursions occurring within a predeterminedtime. The foregoing describes a basic embodiment of the presentinvention. A further improvement of the device of FIG. 1 is shown indashed lines as a feedback circuit 32. Feedback circuit 32 effectivelyvaries the predetermined reference amplitude 23 and 25 in directproportion to changes in the time average of the output signalamplitude.

FIG. 2 is a schematic diagram of the basic embodiment of FIG. 1.Receiver 16 is shown to comprise a transducer 34 which is responsive tovariations in the received acoustic wave to correspondingly vary thebase current applied to the base lead of transistor 36. Transistor 36 isthe amplifier of one stage of the two stage common emitter amplifiershown generally as 38. The combination of transducer 34 and amplifier 38are tuned to the carrier wave frequency, which for the present exampleshall be assumed to be 40 KHZ. The output of the two stage commonemitter amplifier 38 is applied to an emitter follower amplifier 40. Theemitter follower output is an electrical signal representative ofamplitude demodulation of a received wave. The emitter follower 40output is shown to be applied as the input to saturation amplifier 20.Saturation amplifier 20 comprises a first common emitter amplifier,shown generally as 42, and a second common emitter amplifier, showngenerally as 44. The combination of the amplifiers 42 and 44 provides analternating signal representation of the electrical signal output ofreceiver 16. This alternating signal is provided as the input to activefilter 26 which is shown to comprise an active network, shown generallyas 45, and a twin-T network shown generally as 46. Active network 45 isshown to comprise a common emitter amplifier 48 coupled in series withan emitter follower 50 the output of which is the input to twin-Tnetwork 46. The output of twin-T network 46 is fed back via couplingcapacitor 52 to the input of common emitter amplifier 48. The output ofactive filter 26 is provided through coupling capacitor 54 as the inputsignal to alarm and indicator circuit 30. The alarm and indicatorcircuit 30 is shown to generally comprise an emitter follower 56 andintegrator circuit 58, a Schmidt Trigger 60 and a relay output 61. Thepath for charging capacitor 62 of integrator 58 is through transistor64, and resistor 66. The discharge path of capacitor 62 is throughresistor 70. The charge time constant of integrator 58 is chosen so thatrandom, short duration disturbances characteristic of environmentalnoise do not cause an alarm. The discharge time constant is selected tobe much larger than the charge time constant so that a long durationdisturbance characteristic of a valid alarm condition will cause analarm. We have found a ratio of the discharge to charge time constantsof about 30 to be acceptable.

FIG. 3 illustrates a circuit which would provide compensation forvariations in the ambient level of turbulence signals. The connectionpoints A through E of the circuit of FIG. 3 would be connected at thesimilarly identified points in FIG. 2. In addition to providing aslightly different integrator circuit, shown generally as 74, thecircuit of FIG. 3 includes a feedback network shown generally as 76.Integrator 74 is essentially the same as the integrator 58 having acharge path through transistor 64 and resistor 66 to capacitor 62. Theintegrator discharge path is through resistors 80 and 82. Therelationship between the charge and discharge time constants ofintegrator 74 is the same as that previously discussed for integrator58. Feedback network 76, as shown, may conveniently be a Millerintegrator. The transistor 84 of the integrator is shown to have itscollector lead coupled to the output of the first common emitteramplifier of the saturation amplifier 20. The Miller integrator offeedback network 76 timeaverages the charge stored by capacitor 62 ofintegrator 74 to sink current from, i.e., to load, the output of commonemitter amplifier 42 in direct proportion to the charge stored incapacitor 62.

Briefly, the operation of a circuit of FIG. 2 is as follows. Transducer34 receives reflected waves of the carrier waves propagated into thespace to be protected and varies the bias current applied to the base oftransistor 36 in a manner corresponding to variations of the receivedwave. Emitter follower 40 strips the carrier frequency from the outputsignal of amplifier 38 to provide an electrical signal which isrepresentative of the amplitude modulation of the received wave. Theamplifier 20 receives this electrical signal as its input and is driveninto saturation whenever the amplitude of the input signal exceeds apredetermined reference amplitude. Returning to FIG. 1 momentarily, itis seen that when the signal 18 exceeds the reference amplitudeindicated by dashed lines 23 and 25, an alternating signal is producedhaving an excursion (a first excursion) extending between referencelines 27 and 29. On the other hand, if the amplitude of signal 18 isless than the reference amplitude of lines 23 and 25, the excursion ofthe corresponding alternating signal of amplifier 20 is less than thefirst excursion. The alternating signal of amplifier 20 is applied tothe input of active filter 26 which has a pass-band of from about 50 Hz.to 200 Hz. In FIG. 1, the portion of waveform 22 corresponding toperiods t and 1 of the time scale corresponds to a frequency between 50Hz. and 200 Hz. and thus are passed by active filter 26 as an outputsignal. Portions of the waveform 22 of period t and 1 however,correspond to frequencies respectively below and above the passband offilter 26 and thus their passage is essentially blocked.

The active filter output is applied to the base of emitter follower 56the emitter current of which flows through resistor 66 and intocapacitor 62. Schmidt Trigger 60 is coupled between resistor 66 andcapacitor 62 as an amplitude detector to produce an alarm signal byswitching states when the charge stored in capacitor 62 reaches apredetermined level, i.e., the trigger level of the Schmidt Trigger. Thetrigger level is chosen to be sufficiently greater than the quiescentcharge of capacitor 62 that neither a half excursion, nor even severalsuccessive full excursions, of the output signal will raise the chargein capacitor 62 to the trigger level for it has been found thatenvironment noise can generate several successive first amplitudeexcursions within the frequency pass-band characteristic of validalarms. Assuming a carrier frequency of 40 KHL, a frequency of theamplitude demodulated signal (corresponding to an intruder) of Hz. and,to simplify analysis, an idealized waveform, approximately 40 excursionswould be required to raise the quiescent charge (about 1.2 v.) oncapacitor 62 to the trigger voltage (1.5 v.) of Schmidt Trigger 60.Feedback network 32 compensates for variations in the ambient noiselevel which would change the quiescent charge of capacitor 62 (and thusalso change the differential charge required to reach the triggerlevel). As the charge on capacitor 62 increases above the quiescentlevel, the Miller integrator time-averages the charge and sinks or drawscurrent from the output of common emitter amplifier 42 in proportion tothe time-averaged charge. The current drawn off by Miller Integrator 76is thus diverted from the input to common emitter amplifier 44 whicheffectively increases the amplitude of the input signal required todrive amplifier 20 into saturations, i.e., it effectively increases thepredetermined reference amplitude.

Typical values of components of the circuits of FIGS. 2 and 3 are givenin the following table.

R5 1 KO C5 .22 of R6 56 KG C6 .22 f R7 470.0 C7 .22 ,u.f R910KflC8l25p.f,l6v R1047Q C910 f, 16v R11 I KO C10 .047 f R12 3.3 meg!) C11I25 pf, 4 v R13 470 K!) C12 .22 pf R1410KQ C13 .0l5 .f R 47 KO C14 I25pf, 4 v R1610 KO C15 125 M14 v R1715 KO Cl8.1 ,f R18 22.0 C19 .1 f R193.3 KG C20 .22 .f R20 150 K0 C52 l0 .f, 16v R21 2.2 KG C54 .22 pf R22 33K0 C62 125 i4 v R23 22 K9 all transistors GE 2N3394 R24 1009 R25 4.7 KGTransducer 34 R26 56 KG 40 KHz ceramic transducer, R27 4.7 K!)commercial type number R28 10 KO MK-l09 offered for sale by R29 47.0MASSA DIVISION of R30 1 K9 Dynamics Corporation of R31 15 KO America R3215 KO R33 l5 KO. R34 8.2 KO R35 47 K0 R39 1.2m R40 390 R41 820 KO R42 15K0 R66 6800 R70 I00 K!) II. FIG. 3

Resistor Capacitor R8027 KO C88l25p.fl0v R82 47 KO R86 2.2 KG Transistor84 GE 2N3394 It will be appreciated that, while certain specificembodiments have been shown and described, various changes andmodifications may be resorted to without departing from the true spiritand scope of the invention as defined in the appended claims.

What is claimed is:

I. An ultrasonic motion detector device which receives an ultrasonicacoustic wave radiated into a space to be protected at an essentiallyconstant carrier frequency and amplitude and which produces an alarmsignal when the modulations of a received wave are characteristic of analarm condition, comprising:

A. means for producing an electrical signal which is representative ofthe amplitude modulations of a received ultrasonic wave;

B. means for producing an alternating signal in response to saidelectrical signal, said alternating signal having a frequencycorresponding to the frequency of said electrical signal but thedifference between the high and low amplitudes of an excursion of saidalternating signal being a first value when the absolute value of theelectrical signal amplitude is greater than a predetermined referenceamplitude and being another value less than the first value when theabsolute value of the electrical signal amplitude is less than thepredetermined reference amplitude;

C. means for passing said alternating signal as an output signal whenthe frequency of said alternating signal is within a predeterminedpassband;

D. means for producing an alarm signal in response to an output signalwhich corresponds to a train of a predetermined number of alternatingsignal excursions of said first value occurring within a predeterminedtime.

2. An ultrasonic motion detector device according to claim 1 furthercomprising means for effectively varying said predetermined referenceamplitude in direct proportion to the time average of the output signalamplitude.

3. An ultrasonic motion detector device which receives an ultrasonicacoustic wave radiated into a space to be protected at an essentiallyconstant carrier frequency and amplitude and which produces an alarmsignal when the modulations of a received wave are characteristic of analarm condition, comprising:

A. means for producing an electrical signal which is representative ofthe amplitude modulations of a received ultrasonic wave;

B. a saturation amplifier for producing an alternating signal inresponse to said electrical signal, said alternating signal having afrequency corresponding to the frequency of said electrical signal butthe difference between the high and low amplitudes of an excursion ofsaid alternating signal being a first value when the absolute value ofthe electrical signal amplitude is greater than a predeterminedreference amplitude and being another value less than the first valuewhen the absolute value of the electrical signal amplitude is less thanthe predetermined reference amplitude;

C. an active filter for passing the alternating signal as an outputsignal having a pass-band of from about 50 Hz. to 200 l-Iz.; and

D. means for producing an alarm signal in response to an output signalwhich corresponds to a train of a predetermined number of alternatingsignal excursions of said first value occurring within a predeterminedtime, which means comprises an integrator having a charge time constantand a discharge time constant greater than said charge time constant,which integrator is responsive to one-half of each output signalcorresponding to one-half of an alternating signal excursion to store acharge at a rate determined by said charge time constant and responsiveto the other one-half of each output signal to reduce the stored chargeat a rate determined by said discharge time constant and an amplitudedetector responsive to the stored charge reaching a predetermined levelto produce an alarm signal 4. An ultrasonic motion detector according toclaim 3, wherein the saturation amplifier comprises a first commonemitter amplifier which receives the electrical signal as its input,

a second common emitter amplifier coupled in series with the firstcommon emitter amplifier, and

wherein the means for effectively varying the predetermined amplitudecomprises a feedback network for timeaveraging the charge stored in saidintegrator and for loading the output of said first common emitter indirect proportion to said time-averaged charge.

5. An ultrasonic motion detector according to claim 4, wherein saidfeedback network loads the output of said first common emitter amplifierrelatively lightly when said first integrator is storing less than apredetermined charge but abruptly changes to load said output relativelyheavily when the charge stored in said first integrator exceeds apredetermined charge.

6. An ultrasonic motion detector according to claim 4, wherein saidfeedback network is a current sink which draws current from the outputof said first common emitter amplifier in direct proportion to thecharge stored in said first integrator.

7. An ultrasonic motion detector according to claim 5, wherein saidactive filter comprises a third common emitter amplifier having itsinput coupled to the output of said saturation amplifier;

an emitter follower having its input coupled to the output of said thirdcommon emitter amplifier; and

a twin-T filter having its input coupled to the output of said emitterfollower and having its output fed back to the input of said thirdcommon emitter amplifier.

8. An ultrasonic motion detector device which receives an ultrasonicacoustic wave radiated into a space to be protected at an essentiallyconstant carrier frequency and amplitude and which produces an alarmsignal when the modulations of a received wave are characteristic of analarm condition, comprising:

A. means for producing an electrical signal which is representative ofthe amplitude modulations of a received ultrasonic wave;

B. a saturation amplifier for producing an alternating signal C. anactive filter for passing said alternating signal as an output signalhaving a frequency, the periodicity of which is within a predeterminedpass-band, said pass-band having a lower limit which will rejectturbulence type signals and an upper limit which will rejectenvironmental signals like those of a ringing telephone; and

D. an alarm and indicator circuit for producing an alarm in response tosaid output signal, corresponding to a train of said first amplitudealternating signal excursions occurring within a predetermined time.

1. An ultrasonic motion detector device which receives an ultrasonicacoustic wave radiated into a space to be protected at an essentiallyconstant carrier frequency and amplitude and which produces an alarmsignal when the modulations of a received wave are characteristic of analarm condition, comprising: A. means for producing an electrical signalwhich is representative of the amplitude modulations of a receivedultrasonic wave; B. means for producing an alternating signal inresponse to said electrical signal, said alternating signal having afrequency corresponding to the frequency of said electrical signal butthe difference between the high and low amplitudes of an excursion ofsaid alternating signal being a first value when the absolute value ofthe electrical signal amplitude is greater than a predeterminedreference amplitude and being another value less than the first valuewhen the absolute value of the electrical signal amplitude is less thanthe predetermined reference amplitude; C. means for passing saidalternating signal as an output signal when the frequency of saidalternating signal is within a predetermined passband; D. means forproducing an alarm signal in response to an output signal whichcorresponds to a train of a predetermined number of alternating signalexcursions of said first value occurring within a predetermined time. 2.An ultrasonic motion detector device according to claim 1 furthercomprising means for effectively varying said predetermined referenceamplitude in direct proportion to the time average of the output signalamplitude.
 3. An ultrasonic motion detector device which receives anultrasonic acoustic wave radiated into a space to be protected at anessentially constant carrier frequency and amplitude and which producesan alarm signal when the modulations of a received wave arecharacteristic of an alarm condition, comprising: A. means for producingan electrical signal which is representative of the amplitudemodulations of a received ultrasonic wave; B. a saturation amplifier forproducing an alternating signal in response to said electrical signal,said alternating signal having a frequency corresponding to thefrequency of said electrical signal but the difference between the highand low amplitudes of an excursion of said alternating signal being afirst value when the absolute value of the electrical signal amplitudeis greater than a predetermined reference amplitude and being anothervalue less than the first value when the absolute value of theelectrical signal amplitude is less than the predetermined referenceamplitude; C. an active filter for passing the alternating signal as anoutput signal having a pass-band of from about 50 Hz. to 200 Hz.; and D.means for producing an alarm signal in response to an output signalwhich corresponds to a train of a predetermined number of alternatingsignal excursions of said first value occurring within a predeterminedtime, which means comprises an integrator having a charge time constantand a discharge time constant greater than said charge time constant,which integrator is responsive to one-half of each output signalcorresponding to one-half of an alternating signal excursion to store acharge at a rate determined by said charge time constant and responsiveto the other one-half of each output signal to reduce the stored chargeat a rate determined by said discharge time constant and an amplitudedetector responsive to the stored charge reaching a predetermined levelto produce an alarm signal
 4. An ultrasonic motion detector according toclaim 3, wherein the saturation amplifier comprises a first commonemitter amplifier which receives the electrical signal as its input, asecond common emitter amplifier coupled in series with the first commonemitter amplifier, and wherein the means for effectively varying thepredetermined amplitude comprises a feedback network for time-averagingthe charge stored in said integrator and for loading the output of saidfirst common emitter in direct proportion to said time-averaged charge.5. An ultrasonic motion detector according to claim 4, wherein saidfeedback network loads the output of said first common emitter amplifierrelatively lightly when said first integrator is storing less than apredetermined charge but abruptly changes to load said output relativelyheavily when the charge stored in said first integrator exceeds apredetermined charge.
 6. An ultrasonic motion detector according toclaim 4, wherein said feedback network is a current sink which drawscurrent from the output of said first common emitter amplifier in directproportion to the charge stored in said first integrator.
 7. Anultrasonic motion detector according to claim 5, wherein said activefilter comprises a third common emitter amplifier having its inputcoupled to the output of said saturation amplifier; an emitter followerhaving its input coupled to the output of said third common emitteramplifier; and a twin-T filter having its input coupled to the output ofsaid emitter follower and having its output fed back to the input ofsaid third common emitter amplifier.
 8. An ultrasonic motion detectordevice which receives an ultrasonic acoustic wave radiated into a spaceto be protected at an essentially constant carrier frequency andamplitude and which produces an alarm signal when the modulations of areceived wave are characteristic of an alarm condition, comprising: A.means for producing an electrical signal which is representative of theamplitude modulations of a received ultrasonic wave; B. a saturationamplifier for producing an alternating signal in response to saidelectrical signal, having a frequency corresponding to the frequency ofsaid electrical signal, the excursions of which have a first amplitudewhenever said electrical signal exceeds a predetermined referenceamplitude and have a second amplitude less than said first amplitudewhenever said signal is less than said predetermined referenceamplitude; C. an active filter for passing said alternating signal as anoutput signal having a frequency, the periodicity of which is within apredetermined pass-band, said pass-band having a lower limit which willreject turbulence type signals and an upper limit which will rejectenvironmental signals like those of a ringing telephone; and D. an alarmand indicator circuit for producing an alarm in response to said outputsignal, corresponding to a train of said first amplitude alternatingsignal excursions occurring within a predetermined time.